1. Field of the Invention
The present invention relates to a current measuring circuit for measuring discharging current and charging current of a battery, which is selectively connected to a load such as an electronic apparatus or the like and a battery charger.
2. Description of the Related Art
Mobile terminals such as portable telephone sets are generally driven by a battery. Therefore, it is desirable to constantly measure the charged condition of the battery and to charge the battery when the residual capacity of the battery decreases. In a battery such as a nickel-cadmium battery, the voltage does not decrease significantly but the discharging current decreases remarkably when the residual capacity decreases. Accordingly, residual capacity meters adapted to detect the discharging current of a battery to thereby measure the residual capacity of the battery have been in wide use. For example, a discharging current measuring circuit of the prior art is disclosed for example in Japanese Laid-open Patent Publication No. 59-145485. In this reference, a resistor is connected to the positive terminal of a battery and a voltage is directly measured across the resistor through an A/D converter. With such a current measuring circuit, the current value I can be obtained by substituting the potential difference V across the resistor having the resistance value R in Ohm's law I=V/R.
In the prior art current measuring circuit, the discharging current of the battery is restrained by the inserted resistor, and therefore, there have been problems with such prior art current measuring circuits. These prior art current measuring circuits are not suitable for measuring the discharging current of a battery supplying a large current and are unable to measure a wide range of currents.
There is also known such a type of current measuring circuit, which is adapted such that the potential difference across a resistor for current detection is amplified by a differential amplifier before being output to an A/D converter. However, in correctly measuring the discharging current of a battery, an operational amplifier of an expensive low offset/low drift voltage type must be used in the differential amplifier. Hence, there has been a problem with this type in that the current measuring circuit incurs an increase in cost.